Editing Transmitter (section)

===Components===
A practical radio transmitter mainly consists of the following parts:

*In high power transmitters, a [[power supply]] circuit to transform the input electrical power to the higher [[voltage]]s needed to produce the required power output.
*An [[electronic oscillator]] circuit to generate the [[radio frequency]] signal. This usually generates a [[sine wave]] of constant [[amplitude]], called the [[carrier wave]] because it produces the radio waves which "carry" the information through space. In most modern transmitters, this is a [[crystal oscillator]] in which the frequency is precisely controlled by the vibrations of a [[quartz crystal]].  The [[frequency]] of the carrier wave is considered the frequency of the transmitter.
*A [[modulator]] circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter as an electronic signal called the [[modulation|modulation signal]].  The modulation signal may be an [[audio signal]], which represents [[sound]], a [[video signal]] which represents moving images, or for data in the form of a [[Binary numeral system|binary]] [[digital signal]] which represents a sequence of [[binary digit|bits]], a [[bitstream]].  Different types of transmitters use different [[modulation]] methods to transmit information: 
**In an AM ([[amplitude modulation]]) transmitter the [[amplitude]] (strength) of the carrier wave is varied in proportion to the modulation signal.
**In an FM ([[frequency modulation]]) transmitter the [[frequency]] of the carrier is varied by the modulation signal.
**In an FSK ([[frequency-shift keying]]) transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two [[binary digit]]s, 0 and 1. 
**OFDM ([[orthogonal frequency-division multiplexing]]) is a family of complicated [[digital modulation]] methods very widely used in high bandwidth systems such as [[Wi-Fi]] networks, [[cellphone]]s, [[digital television]] broadcasting, and [[digital audio broadcasting]] (DAB) to transmit digital data using a minimum of [[radio spectrum]] bandwidth.  OFDM has higher [[spectral efficiency]] and more resistance to [[fading]] than AM or FM.  In OFDM multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming [[bitstream]] so multiple [[binary digit|bits]] are being sent simultaneously, in parallel.  At the receiver the carriers are demodulated and the bits are combined in the proper order into one bitstream. 
:Many other types of [[modulation]] are also used.  In large transmitters the oscillator and modulator together are often referred to as the ''exciter''.
*A radio frequency (RF) [[amplifier]] to increase the power of the signal, to increase the range of the radio waves.
*An [[impedance matching]] ([[antenna tuner]]) circuit to transform the output [[Electrical impedance|impedance]] of the transmitter to match the impedance of the antenna (or the [[transmission line]] to the antenna), to transfer power efficiently to the antenna.  If these impedances are not equal, it causes a condition called [[standing wave]]s, in which the power is reflected back from the antenna toward the transmitter, wasting power and sometimes overheating the transmitter.
In higher frequency transmitters, in the [[Ultra high frequency|UHF]] and [[microwave]] range, free running oscillators are unstable at the output frequency. Older designs used an oscillator at a lower frequency, which was multiplied by [[frequency multiplier]]s to get a signal at the desired frequency. Modern designs more commonly use an oscillator at the operating frequency which is stabilized by phase locking to a very stable lower frequency reference, usually a crystal oscillator.